The story of medieval Christian Scandinavia, after the various regions convert in the 10th and 11th century, is of dynasties in Denmark, Norway and Sweden struggling to establish stable kingdoms - with sometimes the added ambition of bringing the other two into a unified realm.

The earliest recognizable kingdom is that of Hemming in southern Denmark from 811; but the king's successors fail to hold his territory. Another century passes before the whole of Denmark is united in a single kingdom, under the rule of Harald Bluetooth - who is baptized a Christian in about 960.

In the way of royal converts, he sees this personal event as the conversion of all the Danes (an achievement commemorated in Denmark's famous Jelling Stone).

Harald's son Sweyn extends the Danish kingdom to England in 1013, and his grandson Canute rules an empire which includes Denmark, England and even for a while (1030-1035) the kingdom of Norway.

Norway has only a few years previously become a single kingdom. Olaf II, ruling from 1015 to 1030, unites the whole region under one crown. Sweden achieves similar unity rather later; not until the dynasty established by Birger Jarl in the 13th century does the Swedish kingdom have the stature to match Denmark or Norway.

At various times different regions become dominant within this Scandinavian triangle. Valdemar I and his son Valdemar II extend Danish influence along the Baltic coast between 1169 and 1222. From about 1240 Haakon IV gives Norway an expansive period, asserting control over distant Iceland and Greenland. In 1323 Sweden is strong enough to incorporate much of Finland, agreeing a boundary in that year with the Russians of Novgorod.

Meanwhile, incessantly, the rulers of the Scandinavian kingdoms engage in two closely related methods of affecting the balance of power among themselves. They go to war against each other. And they marry each other's daughters. One such marriage, in 1363, leads at last to the union of the three crowns.

Union of the crowns: 1363-1523

Margaret, who unites the three crowns of Scandinavia, is the daughter of Valdemar IV, king of Denmark. In 1363, at the age of ten, she is married to Haakon VI, the 23-year-old king of Norway. Seventeen years later her father and her husband are dead, but she has a young son, Olaf. She secures his acceptance as king of both Denmark and Norway, and rules very effectively in his name.

In 1387 the young king dies. Margaret's authority is now such that she is accepted in her own right, in 1388, as the 'sovereign lady and ruler' of both countries. In that same year she is given the opportunity to add Sweden to her portfolio. The Swedish nobles, accustomed to electing their kings, are discontented with the present incumbent. They enlist Margaret's help.

Before marching against the present king (Albert of Mecklenburg), Margaret declares her terms. She is to be sovereign lady and ruler of Sweden as of the other kingdoms (the phrase effectively means regent) and the Swedes are to accept her choice of the king to succeed her. With this agreed, she defeats Albert in battle in 1389 and takes control.

Stockholm holds out against her (it is virtually an independent city run by the German merchants of the Hanseatic League). But in 1398, in return for confirmation of the league's commercial privileges, it too becomes part of her domain. The three Scandinavian countries are now a united regency. And the regent has already selected an infant king, to create a united kingdom.

In 1389 Margaret declares that her 8-year-old great-nephew Eric of Pomerania (grandson of her elder sister) is king of Denmark, Norway and Sweden. The three realms become formally united when he is crowned at Kalmar in 1397. Margaret is officially regent only until Eric is declared of age (in 1401), but she continues to rule in his name - as effectively as ever - until her death in 1412.

In subsequent decades Eric follows the same policies as his great-aunt, but he is unable to hold the union together. Uprisings against him in all three kingdoms lead to his deposition in Denmark and Sweden in 1439, followed by Norway in 1442.

For almost another century there are attempts, sometimes briefly successful, to restore the union of the three realms under a single king. The last such king is Christian II, who rules in Denmark and Norway from 1513. He has to fight for his Swedish crown. After three years of war he takes Stockholm, in 1520, but it proves a brief triumph. He is crowned on November 4. Four days later a massacre in Stockholm prompts the uprising which results in the Vasa dynasty and an independent Sweden.

Christian loses his other two crowns, of Denmark and Norway, in 1523. From now on, although Norway does not achieve independence until 1905, the story of each Scandinavian country is clearly distinct.